Maize kernels contain a commercially useful starch, which is a combination of amylopectins and amyloses. Amylose and amylopectin are both polymers of alpha-d-glucopyranose. The polymers differ only in the kind of linkages between glucose residues. Amylose is a linear polymer that has alpha-(1-4) links between adjacent glucose residues, and amylopectin is a branched polymer that contains some alpha-(1-6) links as well. The long amylose and amylopectin polymers store glucose in a semi-insoluble state. Glucose can be obtained by the plant cell by selective degradation of the polymers by specific enzymes.
Maize starch has many food-related applications, such as corn syrups, maltodextrins, dextrins and high fructose corn syrup. Although natural maize starch is useful, much of the starch used commercially today is either chemically or enzymatically modified. The usual purpose of chemical and enzymatic modification of maize starch is to alter the viscosity, texture, heat stability, freeze-thaw stability and other functional qualities of the starch.
There is keen interest today in providing natural maize starches which have the same properties as chemically-modified or enzymatically-modified starches. Use of natural starches eliminates the need for further processing of maize starches. This processing can be quite expensive. Additionally, consumers prefer "natural" food ingredients.
Conventional maize breeding can produce maize with altered corn starch. It was recognized in the 1920's that the "waxy" maize mutant produced altered starch, and maize derived from this mutant was commercially grown in the 1940's specifically for its starch content. Waxy maize produces starch that is nearly pure amylopectin. Maize varieties producing starch with a high amylose content were successfully developed in the 1950's. A great deal of emphasis from the 1950's through until the 1980's has been on improving the techniques for chemical and enzymatic modification of the maize starch.
Identification of a number of mutant maize genes has been the focus of much of the work directed towards creating new varieties of maize for altered starch. Workers have concentrated on the wx (waxy) gene of waxy maize and the ae (amylose extender) gene of high amylose maize. Other genes of interest to maize breeders have been the du (dull), sul (sugary 1), su2 (sugary 2), f1 (flouryl), sh2 (shranken 2), and h (hard or horny) genes. Combinations of these genes in maize have produced starches with a wide range of amylopectin-amyloses ratios, molecular weights and structural characteristics.
Viscosity and viscosity stability are primary criteria for starch applications in food systems. Currently, there is only one available untreated starch with the ability to cool without increasing in viscosity. That instance is a double mutant of dull and waxy, which is limited in use because of its reduced productivity. A starch with this property could be used in certain food products that are heated (for example, baby foods) without the necessity of chemical modification.